The use of lasers to cut integrated circuits is well-known in the art. In existing methods, a laser is directed onto a cut-link segment of the circuit. The laser supplies sufficient heat to vaporize the cut-link, thereby severing the cut-link. In many cases such as in a typical microchip, the circuit is coated with a passivation layer to protect the circuit from oxidation. To allow the cut-link to vaporize, the passivation layer is usually etched away from the site above the cut-link to expose the cut-link to the atmosphere in advance of cutting.
Lasers are used to cut circuits for a variety of purposes including the avoidance of defects, the replacement of defective circuits with redundant circuits, and the programming of circuits. The practices used to achieve each of these objectives are described, below.
Lasers have been used to avoid defects by circumventing a short through laser-induced cuts. This practice is described in U.S. Pat. No. 4,259,367, issued to Doherty, Jr. The circuit is repaired by using a laser to cut the lines connected to the short to thereby isolate the short. Patch lines may then be used to reestablish severed connections.
The use of redundant circuitry has become an industry standard for memory chip fabrication. Redundant circuitry in a chip is typically supplied in the form of identical segments of circuitry occupying remote areas on the chip. If a segment is flawed, that segment is disconnected and replaced by activating its redundant counterpart elsewhere on the chip.
Finally, circuits may be programmed by cutting lines from a generic, repeating pattern to disconnect unwanted or unnecessary segments and thereby produce the desired pattern through a process of reduction. Circuitry produced by this method may extend across a plurality of levels as well. One example of multi-level circuit design by laser-induced cutting is disclosed in U.S. Pat. No. 4,720,470, issued to Johnson.
Typically, the cut-link has taken one of two forms. First, the cut-link is often an undistinguished segment of a line in the circuit, where the width of the cut-link is equal to the width of the lines to which it is conductively coupled. A cut-link exhibiting these characteristics is illustrated in FIG. 1 and disclosed, for example, in the following U.S. Pat. Nos.: No. 4,853,758, issued to Fischer, and No. 5,589,706, issued to Mitwalsky et al. The second common form is that of a "dog bone," which is illustrated in FIG. 2. This design is disclosed, for example, in the following U.S. Pat. Nos.: No. 4,748,491, issued to Takagi, and No. 5,374,590, issued to Batdorf et al. In this design, the cut-link 20 is narrower than the lines 21, 22 to which it is connected.